Vapour pressure is a well-known parameter related to the volatility of a substance. The vapour pressure is the pressure of a gaseous substance in equilibrium with the condensed substance in a closed system at a fixed temperature.
Vapour pressure can be measured using many standard methods. Examples of standard methods for measuring hydrocarbon vapour pressures include ASTM D6377: Determination of Vapour Pressure of Crude Oil: VPCRx (Expansion Method); ASTM D6378: Determination of Vapour Pressure (VPX) of Petroleum products, Hydrocarbons, and Hydrocarbon-Oxygenate mixtures (Triple Expansion Method); and ASTM D6897: Vapour Pressure of Liquefied Petroleum Gases (LPG) (Expansion Method). Results from vapour pressure analyses can also be correlated back to Reid Vapour Pressure and others by the use of published and accepted correlation factors. Correlated test methods include ASTM D323: Standard Test Method for Vapour Pressure of Petroleum products (Reid Method); ASTM D4953: Standard Test Method for Vapour Pressure of Gasoline and Gasoline-Oxygenate Blends (Dry Method); ASTM D5190: Standard Test Method for Vapour Pressure of Petroleum Products (Automatic Method); ASTM D5191: Standard Test Method for Vapour Pressure of Petroleum Products (Mini Method); ASTM D5188: Standard Test Method for Vapour-Liquid Ratio Temperature Determination of Fuels (Evacuated Chamber Method); and ASTM D5482: Standard Test Method for Vapour Pressure of Petroleum Products (Mini Method-Atmospheric).
Methods such as those above have typically been carried out in a system in which a sample is placed into a sealed chamber and a piston withdrawn so as to expand the volume of the chamber. Because the chamber is sealed, the expansion results in a known volumetric ratio of liquid to vapour. After a pause to allow the system to equilibrate, the pressure in the chamber is measured. From the known vapour/liquid ratio and the measured pressure, the vapour pressure can be calculated. The chamber may be expanded once or multiple times, depending on the method in question. The movement of the piston, and hence the expansion of the chamber, is controlled by using a stepper-motor to drive the piston. Stepper-motors offer control of motion in small, discrete steps. While this can produce fine control of the motion of the piston, it would be desirable to provide more precise determination of the motion so as to produce a more accurate measurement.
Vapour pressure measurements of hydrocarbons may be carried out in chemical plants such as refineries. The measurements can be carried out by taking a sample to a laboratory but doing so results in a delay in obtaining the data. A method of taking online measurements is therefore desirable. However, refineries often include zones with potentially explosive atmospheres so online measurements need to be safe for use in such zones.
The present invention seeks to mitigate the above-mentioned problems. Alternatively or additionally, the present invention seeks to provide improved apparatus and methods for measuring the vapour pressure of hydrocarbons.